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3 Design of a Monitoring System for Determining Subsea Wellhead Fatigue Utilisation Richard Baker Senior Engineer, 2H Offshore Engineering OI2016, London, March 2016

4 Agenda Why monitor subsea wellhead fatigue? What to measure? Key monitoring system design considerations Design considerations in more detail and case studies Summary Questions 3 of 24

5 Why Monitor Wellhead Fatigue? Drilling riser connected to the well for longer periods Poor fatigue design of older wellheads Increased loading from newer vessels with larger BOP stacks Analysis is generally conservative and requires validation Objective To support the use of reduced safety factors and extend life, through: Defining average conservatism Reducing uncertainty (improve confidence) 4 of 24

6 What to Measure? We can t measure fatigue The closest we can get is strain Practical difficulties mean that motion measurements are generally used Transfer functions are needed to convert motion measurements into wellhead loads 5 of 24

7 Converting Measurements to Fatigue Damage Angular rate, acceleration from sensor on BOP stack Bending moment at fatigue hotspot Bending stress Cycles to failure Transfer function Geometry Statistical fatigue information (SCF, S-N design curve) Cycle duration X Riser Lower Flex Joint BOP stack Mudline Wellhead Life 6 of 24

8 Key Monitoring System Design Considerations 1. How critical is the wellhead system? 2. Sensor capabilities 3. Understanding of response 7 of 24

9 Key Monitoring System Design Considerations Operator Hardware supplier CRITICALITY SENSOR CAPABILITIES Objective UNDERSTANDING SYSTEM RESPONSE Analysis and design team 8 of 24

10 Key Monitoring System Design Considerations CRITICALITY SENSOR CAPABILITIES UNDERSTANDING SYSTEM RESPONSE 9 of 24

11 Criticality - how critical is the system that s being monitored? Primary interest of operator CRITICALITY May not require monitoring! May require constant monitoring Criticality affects the key early UNDERSTANDING SYSTEM RESPONSE decision: what type of monitoring system: Real time, Quasi-real time or standalone 10 of 24

12 Types of Monitoring System Hardwired (Real-time) Acoustic (Quasi Real time) Stand-alone 11 of 24

13 Monitoring System Pros and Cons Factors affecting complexity, redundancy, cost Hardwired (Real-time) Acoustic (Quasi Real time) Stand-alone + Continuous logging *Potentially flexible installation. *Quick to mobilise *Good as redundancy - *Slow to implement *Costly *Doesn t remove requirement for ROV. *Opportunity for accelerated data acquisition. *Lengthy downtime *No failure feedback 12 of 24

14 Importance of understanding response CRITICALITY SENSOR CAPABILITIES UNDERSTANDING SYSTEM RESPONSE Understanding system criticality important in determining type of monitoring system Understanding system response required to define Sensor specifications KPIs 13 of 24

15 Sensor Specification Recommendations Sensor type Sensor range Sensor resolution Sensor accuracy Sensor frequency band Sensor noise floor The smallest measurement that can be taken with certainty

16 Case Study: Effect of Response on Sensor Specifications CRITICALITY SENSOR CAPABILITIES Operator engages vendor directly Limited system response understanding Sensors installed on BOP stack X Y B A Mudline

17 Case Study: Effect of Response on Sensor Specifications RMS acceleration to achieve 5-year life of LPH weld Factor = 3 RMS acceleration to achieve 5-year life of top up pipe bracket weld More precision required for worse fatigue detail 16 of 24

18 Case Study: Effect of Response on Sensor Specifications CRITICALITY SENSOR CAPABILITIES Effect of limited response understanding at outset of project? B A X Y 17 of 24

19 Case Study: Effect of Response on Sensor Specifications CRITICALITY SENSOR CAPABILITIES Effect of limited response understanding at outset of project? UNDERSTANDING SYSTEM RESPONSE Significant work required to make use of data Costly and inefficient Engage design and analysis team early Clear communication channels required B A X Y 18 of 24

20 Importance of Feedback from Monitoring System CRITICALITY SENSOR CAPABILITIES Uncertainties remain Still can't meet the monitoring objective even with raw sensor data UNDERSTANDING SYSTEM RESPONSE Feedback from monitoring system required to meet monitoring objective Often overlooked when using automated modules 19 of 24

21 Importance of Feedback from Monitoring System Angular rate, acceleration from sensor on BOP stack Transfer function Bending moment at fatigue hotspot Geometry - Well understood Bending stress Statistical fatigue information (SCF, S-N design curve) Cycles to failure - Well understood Cycle duration - Well understood Life X Mudline 20 of 24

22 Transfer Function Variability Frame and stack interface and stiffness A Other considerations Top tension and mud weight Wellhead interface, internal TOC Soil stiffness, scour, external TOC X B BOP hydrodynamic properties Template stiffness Mudline Calibration Learn of transfer more function at required based on monitoring data 21 of 24

23 Case Study: Importance of Feedback from Monitoring System Uncalibrated Fatigue Estimate Fatigue Comparison: Analysis x2 Motion Measurements x of 24

24 Case Study: Importance of Feedback from Monitoring System Calibrated Fatigue Estimate Fatigue Comparison: Analysis x1.3 Motion Measurements x of 24

25 Summary Wellhead fatigue is a concern for older subsea wellheads accessed by newer drilling vessels with larger BOP stacks Understanding of system response, criticality, and sensor capabilities are key aspects of good monitoring system design Calibration of analytical model fundamental in making correct interpretation of wellhead fatigue from monitoring data Hardware offering alone not sufficient to measure fatigue damage: Basis for criticality Transfer function requires simulation and feedback from sensors 24 of 24

26 Questions? D: +44(0)

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